1. Field of the Invention
This invention relates to an aqueous coating resin composition mainly for use in forming anticorrosive coatings, which comprises an aqueous dispersion of a resin containing in the molecule at least 2 epoxy groups and a curing agent composition including a compound having a chelate-forming capacity.
2. Description of the Prior Art
Vehicles now used for paints include a variety of resins, for example, unsaturated fatty acids, alkyd resins, olefin type resins, diene type resins, acrylic resins, polyester resins, epoxy resins, urethane resins and copolymers of these resins. Among them, epoxy resins are excellent in physical properties (adhesion to substrates and hardness) and chemical properties (resistances to chemicals) and the demand for epoxy resins as vehicles for paints has been increasing.
Recently, it has become an important requirement that paints should not be dangerous or harmful. Namely, paints are required to have a much reduced inflammability and no toxicity to the human body.
As one paint meeting such requirements, there has been developed a so-called aqueous paint comprising an epoxy resin dispersed, emulsified or dissolved in water. Coating epoxy resin compositions of this type are disclosed in, for example, the specifications of U.S. Pat. No. 2,811,495, U.S. Pat. No. 2,899,397, U.S. Pat. No. 3,324,041, U.S. Pat. No. 3,355,409, U.S. Pat. No. 3,449,281 and U.S. Pat. No. 3,640,926.
A variety of other resin compositions have also been investigated and developed in the art as aqueous paints.
However, these known aqueous coating resin compositions are still inferior to solvent type coating compositions with respect to such properties as moisture resistance, corrosion resistance and physical properties.
Various attempts have heretofore been made to reduce or overcome these shortcomings. For example, addition of rust inhibiting pigments such as lead cyanamide, lead suboxide, basic lead chromate, red lead, strontium chromate and zinc chromate has been proposed, but use of these pigments is not desirable because these pigments readily cause environmental pollution.
Further, none of the known aqueous coating resin compositions are completely satisfactory in various physical properties represented by the adhesion under a high humidity condition.
An aqueous epoxy resin paint causing no environmental pollution, which has properties comparable or superior to properties of heretofore used solvent type paints containing large quantities of organic solvents, has not yet been developed in the art.
Further, attempts have been made in the past to improve physical properties of coatings by incorporating into a paint a chelate-forming compound such as a polyhydric phenol, e.g., pyrogallol, a phenol carboxylic acid, a chromium-containing complex salt, a phthalocyanine, a pyridine, a derivative thereof or the like and causing a chelating reaction between the coating and the surface of an iron substrate.
As an old instance of a paint of this type, there is known a solvent type resin formed by merely incorporating tannin in a linseed oil type resin or a drying oil (see the specifications of British Pats. No. 826,564 and No. 826,566).
Later, R. N. Faulkner et al. developed a one-pack solvent paint formed by introducing catechol, pyrogallol, gallic acid or gallic acid ester in the form of a covalent bond into a vegetable oil, a fatty acid ester, an alkyd resin, a vegetable oil-modified epoxy ester resin or a vegetable oil-modified polyamide resin by utilizing a catalyst such as a metal alkoxide. Paints of this type are disclosed in, for example, the specifications of British Pat. No. 1,045,118 and U.S. Pats. No. 3,304,276 and No. 3,321,320, and Journal of the Oil and Colour Chemists' Association, 50, 524 (1967) published by the Oil and Colour Chemists' Association. Further, the specification of British Pat. No. 1,114,400 discloses a composition formed by reacting a styrene-allyl alcohole copolymer with a gallic acid ester.
Chelate-forming compositions including an epoxy resin are also known in the art. For example, there are known chelate-forming resins formed by modifying a part of epoxy groups with a monobasic fatty acid and reacting the remaining epoxy groups with a fatty acid derivative having a chelate-forming capacity and containing in the molecule at least two adjacent phenolic hydroxyl groups and one free carboxyl group (see Japanese Patent Publication No. 2439/73); compositions comprising a chelate-forming epoxy-polyamide resin formed by reacting residual epoxy groups of a reaction product formed between an epoxy resin and a polyamide resin having an amine value of up to 10, with gallic acid and incorporating therein, a phosphoric acid derivative (Japanese Patent Publication No. 17443/73); and a one-pack type or two-pack (an amine type curing agent being used) paint comprising a partially esterified product obtained by reacting a part of epoxy groups of an epoxy resin with salicylic or gallic acid or its ester, an epoxy, vinyl or fluoroethylene resin and an organic solvent as a diluent for the foregoing resin components (Japanese Patent Publication No. 4811/74 and Japanese Patent Application Laid-Open Specifications No. 56226/73, No. 56228/74, No. 122538/74 and No. 122597/74).
In each of the foregoing known paints, an organic solvent is used, and hence, they are still insufficient with respect to safety and prevention of environmental pollution. Further, when ingredients of these paints are examined, it is seen that in each of these known paints, the epoxy resin which is the main ingredient, is used in the modified state. In other words, each of these known compositions is a one- or two-pack paint formed by reacting all or a part of epoxy groups in the epoxy resin as the main ingredient with a chelate-forming compound, and therefore, excellent properties inherent in the epoxy resin are drastically lowered.
Still further, in the foregoing conventional chelate-forming solvent type paints, in order to promote the chelating reaction, it is necessary to incorporate polar solvents such as alcohols or chelating reaction promotors such as ferrous chloride, ferric chloride, organic acids and inorganic acids.
Aqueous paints having a chelate-forming capacity are also known in the art. For example, there can be mentioned a paint comprising as a vehicle an epoxidized styrene-butadiene copolymer (Japanese Patent Application Laid-Open Specification No. 8598/74); and a composition comprising an acrylic copolymer or styrene-butadiene copolymer emulsion and a gallic acid ester incorporated in the emulsion (Japanese Patent Publication No. 14412/73). Each of these compositions is defective in that a coating having a sufficient corrosion resistance cannot be obtained.